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Pelican Technical Article:

Thermostat Replacement

Kerry Jonsson

Time:

1 hour1 hr

Tab:

$75

Talent:

**

Tools:

Flat head screwdriver, T10 Torx socket with ratchet

Applicable Models:

Mercedes-Benz W211 (2003-09)

Parts Required:

Thermostat

Hot Tip:

Work on an engine that is cool to the touch

Performance Gain:

Stop overheating from a thermostat that is stuck closed and improved heater output

Complementary Modification:

Replace hoses

Engineers design an engine to run optimally within a specific temperature range. Hopefully this is where the car's engine temperature spends most of the time while being driven. This is typically while the engine is at normal operating temperature. An ice cold engine is too cold to run efficiently. Components in the engine need to expand with the heat generated by the combustion process. Rings and pistons expand and form a better seal with the cylinder walls. Valves expand in the cylinder head creating a better seal with the valve seats. Once the engine is up to temperature all the components in the engine are working at their best. If the engine is too cold or too hot this will only hurt the engine's efficiency.

A cooling system is designed to remove excess heat from the engine and transfer that heat to the coolant. Coolant is circulated to a radiator where it is cooled as it passes through the thin fins. This cooled coolant is then returned to the engine block to absorb more heat and the process is repeated. Since your car may be stuck in traffic for hours on a hot summer day or you may be cruising along at 75 mph in the dead of winter, something needs to regulate the coolant flow to keep the temperature of your engine within its operating range. This is the job of the thermostat.

Your thermostat has a tough job. In the summer the thermostat must open fully as the engine reaches operating temperature. This needs to happen whether the ambient temperature is only a balmy 60 degrees Fahrenheit to over a 100 degrees Fahrenheit. In the winter months the thermostat needs to stay closed longer to allow the engine to build up temperature quickly for both the proper engine operating temperature and quicker heater output of the climate control system. If you are driving in the winter months the thermostat may close a little to reduce coolant flow and keep the engine temperature within the proper range and in the summer in traffic it must open fully to allow for maximum coolant flow and maximum cooling. Over time the thermostat can become weak. There is a spring in the thermostat that reacts to temperature allowing the thermostat to open and close. If this spring becomes weak it will open easier allowing engine temperature to run cooler than it is designed. Corrosion can also cause a thermostat to stick closed causing "overheating". This is why you should flush your coolant at specific maintenance intervals. In this tech article we will go over how to replace your thermostat.

The thermostat can reach over 212 degrees on a fully warmed up engine. Do not attempt to work on an engine that is not cool to the touch. Hot coolant can cause severe burns. Wait until the engine is fully cooled off before attempting repairs.

This picture illustrates underneath the car on the left front side of the engine compartment.
Figure 1

This picture illustrates underneath the car on the left front side of the engine compartment. Locate the radiator drain plug (green arrow).

Place a drain pan underneath where you are working.
Figure 2

Place a drain pan underneath where you are working. Rotate the drain plug clockwise and thread it all the way out. You will not be able to remove it from the radiator but coolant will start to drain out as you loosen the drain plug.

This picture illustrates the engine compartment towards the front.
Figure 3

This picture illustrates the engine compartment towards the front. Using a flat head screwdriver loosen the radiator hose clamp.

Twist the hose first to break free the corrosion and remove the hose from the thermostat housing neck.
Figure 4

Twist the hose first to break free the corrosion and remove the hose from the thermostat housing neck.

Using a T10 Torx socket with ratchet remove the fastener on the left side of the thermostat.
Figure 5

Using a T10 Torx socket with ratchet remove the fastener on the left side of the thermostat. You can remove the serpentine belt if you want. It will give you more room to work but you do not need to remove it to do this job.

Using a T10 Torx socket, remove the fastener on the right of the thermostat.
Figure 6

Using a T10 Torx socket, remove the fastener on the right of the thermostat. Once again you can remove the serpentine belt if you want. It will give you more room to work but you do not need to remove it to do this job.

Using a long flathead screwdriver, lever the corner (green arrow) of the thermostat housing from the engine.
Figure 7

Using a long flathead screwdriver, lever the corner (green arrow) of the thermostat housing from the engine.

You can also use a flathead screwdriver to lever off the opposite side (green arrow) of the thermostat housing.
Figure 8

You can also use a flathead screwdriver to lever off the opposite side (green arrow) of the thermostat housing.

Pull the thermostat out of the block.
Figure 9

Pull the thermostat out of the block. You do not need to remove the serpentine drive belt to do this job. You can just flex it out of the way. We have removed it for photographic purposes. Remove the O-ring seal (red arrow), be sure the O-ring bore is clean before installing a new one. Fit the new O-ring seal inside the thermostat opening in the block. Fit the thermostat housing and tighten down the two fasteners. Install the upper radiator hose to the thermostat housing neck and tighten the hose clamp. Install the serpentine belt if you removed it to do this job. Fill the cooling system according to our tech article on draining, filling and bleeding the cooling system. Start the vehicle and let it warm up. Verify the temperature gauge goes over 80 degrees Celsius with the engine fully warmed up.

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